This study is part of TINN 'Treat Infections in Neonates' a comprehensive project that is evaluating the safety of ciprofloxacin and how it is tolerated by neonates. Ciprofloxacin is an antibiotic that has been used for many years in newborn babies and infants less than 3 months old to treat bacteria that are resistant to other antibiotics. Ciprofloxacin is unlicensed for this age group. The European Medicines Agency and WHO have prioritised research about this drug. The TINN consortium aims to conduct the work required for a license (marketing authorization) in newborn babies and infants less than 3 months old. The aim of this study is to describe how newborn babies and young infants deal with this medicine. Blood samples will be taken at the beginning of the course of antibiotics and at the end. A minimal amount of blood will be required (less than half a teaspoon) in total and collected by staff experienced in blood sampling from neonates in a way that causes the least disruption to the baby. The levels of ciprofloxacin in the blood will be measured. The levels will be used to work out how quickly the body gets rid of the medicine. This will allow recommendations about the best dose and how often the medicine should be given.